Tocopherol compounds, also designated as Vitamin E, are the active components of certain vegetable oils. Vitamin E activity refers to the physiological activity of this group of nutrient materials. Materials having Vitamin E activity all belong to a distinct series of compounds which are all derivatives of chroman-6-ol. These compounds are all tocol derivatives having an isoprenoid C.sub.16 side-chain, including those compounds having an unsaturated C.sub.16 side-chain. The term "tocol" is used to mean 2-methyl-2-(4', 8', 12'-trimethyltridecyl)chroman-6-ol. Alpha-, beta-, gamma- and delta-tocopherols are of primary importance for Vitamin E activity, and are commercially isolated from various natural sources. Also important are the enols such as tocomonoenols, tocodienols and tocotrienols which are tocopherol compounds having an unsaturated side-chain. These unsaturated compounds also exhibit Vitamin E activity. Within this description therefore, the terms "tocopherols", "tocopherol compounds" and "tocopherol homologues" are understood to include such unsaturated toco-enol compounds.
Tocopherols, found widely distributed in organic substances, occur in highest concentrations in cereal grain oils, principally in wheat and corn oils, and also in barley and rye. They are also found in vegetable oils such as safflower, soybean, peanut, cottonseed, linseed, sunflower, rapeseed, palm and in other vegetable sources.
In general, tocopherols are widely used for their antioxidant and Vitamin E activity. They are of great value in foods, feeds and medicines for these reasons. Frequently, mixtures of tocopherols are isolated, purified, and methylated to upgrade Vitamin E activity.
Those tocopherol homologues having an unsaturated C.sub.16 side-chain such as tocotrienol can be hydrogenated and then the beta, gamma, and delta isomers can be upgraded to alphatocopherol Vitamin E activity.
Natural vegetable oils contain small amounts of tocopherols. Such oils as wheat germ oil, soybean oil, and cottonseed oil are considered to be the best sources of Vitamin E. It is desirable for commerical purposes to separate and concentrate tocopherol homologues and to devise methods for separating impurities from tocopherols so that they may be employed for their anti-oxidant and Vitamin E activity.
One method currently used for purifying tocopherols and for the preparation of tocopherol concentrates involves distillation. Distillation, however, fails to separate tocopherols from co-boiling impurities. One method for the preparation of tocopherol concentrates using distillation is reported in U.S. Pat. No. 4,454,329. This distillation process for the purification of tocopherols calls for the esterification of free fatty acids which are present in the feed materials with the tocopherols. The esterified mixture is then distilled and the tocopherols are collected as a distillate. Thus, in this process the fatty acids are esterified and then separated from the tocopherols.
It is worthwhile to note that tocopherol homologues have a phenolic hydroxy group and can therefore be esterified using an acid reactant. Japanese Patent Publication No. 55 (1980-38347) teaches that a borate ester of high reactivity is formed between boric acid and delta-tocopherol. In accordance with this reference, however, the borate delta-tocopherol ester is used as a methylation intermediate for the formation of beta-tocopherol. The intermediate borate delta-tocopherol ester is methylated by a reaction with hexamethyltetramine in the presence of an organic acid catalyst. According to this Japanese publication, the delta-tocopherol ester is a highly reactive intermediate which directs the placement of a methyl group to form beta-tocopherol instead of gamma-tocopherol.
In accordance with the instant invention, borate tocopherol esters can also be formed from the alpha-, beta-, and gamma-tocopherol homologues. The borate esters formed from tocopherol homologues are, moreover, sufficiently stable to be used for the purification of tocopherols. It is therefore an object of the instant invention to provide a process for tocopherol purification using the capacity of alpha-, beta-, gamma-, and delta-tocopherol homologues to form stable borate tocopherol esters. It is a further object of the instant invention to provide a method whereby impurities coboiling with tocopherols can be separated from tocopherol homologues. Other objects will become apparent as this description proceeds.